Connectors for ladders from the old steckcastles do exist, they came with some construction sets in the 70's and 80's... so very long ago. I'm not sure if they still can be found in other sets , as far as I know they don't come in any medieval sets.
yes, they didn't come with the castles but with modern sets, like construction, fire department or such
Here is a set from the 70's they came in, you can see them attached to the ladders:http://www.playmobil.de/on/demandware.store/Sites-DE-Site/de_DE/Product-Detail?pid=3403-A&cgid=1976